1. Field of the Invention
The present invention concerns a detonation arrestor device for bulk explosive materials transfer.
2. Description of the Prior Art
The presence of a detonator at the bottom of a borehole during bulk explosive transfer increases the risk of unintentional explosion. To limit the consequences of any such explosion following electrical disturbances (storms, etc) for example, it is important that any detonation started in the borehole (which can contain as much as several hundred kilograms of explosive in the case of open cast mining) cannot propagate through the loading hose as far as the stock of bulk explosive, generally consisting of one or more storage tanks, the total quantity of which on the same site may be several tons, explosion of which could have catastrophic consequences.
Given the high detonation velocity in a cylindrical charge of explosive (several kilometers per second) and the destructive effects of the shockwave due to the detonation, implementing a detonation arrestor system by simple mechanical means is no simple matter.
The solution currently adopted for limiting the risk of detonation propagating through the loading hose consists in limiting the diameter of the hose to a value below the critical detonation diameter of the pumped explosive. The critical detonation diameter of an explosive is a value of the diameter of a cylindrical charge of that explosive below which detonation is incapable of propagating through the charge.
This solution has been adopted in French regulations governing the pumping of explosives in mines and quarries: with a detonator at the bottom of the borehole, the diameter of the hose for loading explosive by pumping must be limited to 25 mm, a value which is in theory below the critical detonation diameters of all explosives loading of which by pumping is authorized in France.
Limiting the diameter of the hoses to 25 mm significantly reduces the pumping rate and commensurately increases the loading time. This increases the time for which personnel are exposed to explosive hazards and may even call into question the economic advantages of bulk transfer of explosives by pumping, especially in the case of blasting deep vertical open cast mines with the explosion initiated from the bottom of the borehole.
In the case of explosives that can be pumped, which are slurry and emulsion explosives, for example, the choice of the critical detonation diameter as the criterion for limiting the hose diameter is disputable: the value of the critical detonation diameter is an intrinsic characteristic of any given explosive and is determined under specific experimental conditions (lightly filled cartridge, standardized density, etc); this is generally done "once and for all" when the product is being examined for approval. It is not certain that this characteristic is representative of the explosive under normal utilization conditions.
Specifically, the confinement due to the reinforced rubber of the loading hoses may slightly reduce the value of the critical detonation diameter. This parameter may also depend closely on the density of the explosive under its conditions of utilization, however. The density of pumpable explosives may vary significantly with:
the pressure to which the product is subjected in the hose during pumping, this effect being generally in the direction of increased safety, with pressures of the order of a few bars as routinely obtained at pump outlets being sufficient to desensitize the explosives, PA0 commercial imperatives which require manufacturers to regulate the density of their product, which is done by mechanical means (stirring to procure aeration) or chemical means (variation in the quantity of lightening or emulsifying agent).
In the final analysis, the current rule limiting the diameter of pumping hoses not only has major disadvantages from the point of view of practical pumping conditions, but also fails in certain cases to prevent propagation of detonation from the bottom of the borehole to the explosive materials storage tanks.
To alleviate these disadvantages, whilst tolerating fluctuations in the critical diameter of the explosive materials transferred and without affecting the transfer rate, an object of the invention is to arrest detonation between an explosive materials transfer system and a storage tank of the explosive materials.